1. Field of the Invention
The present invention relates to an image sensor and a method for fabricating an image sensor, and more particularly to a back-side illumination image sensor and a method of fabricating a back-side illumination image sensor.
2. Description of the Prior Art
As the pixel size of a complementary metal-oxide-semiconductor image sensor (CMOS image sensor, CIS) grows smaller, the degradation resulting from certain factors such as quantum efficiency, cross-talk and dark current in a sensor array also becomes significant. Regarding a conventional image sensor such as a front-side illuminated sensor, a microlens of each pixel sensor is fabricated on a front side of a substrate. Therefore, the incident light has to travel through dielectric layers between circuitry formed by metal layers to arrive at a photodiode otherwise it may be reflected or absorbed by metal or any other reflective material. Since the traveling path of light cannot be blocked by metal or any other kind of reflective material, the fill factor is limited.
In order to enhance the compactness of an image sensor, a back-side illumination (BSI) image sensor was introduced. In a BSI image sensor, the incident light is projected from a back side of a substrate of the image sensor, and therefore the front side of the substrate can be reserved for circuitry of various functions. With the utilization of the BSI technology, there are more and more available areas on the front side of the substrate for building various functional circuits. Furthermore, in the BSI image sensor, the photodiode of the BSI image sensor is utilized for transforming incident light into electrical signals. Thus, the transforming efficiency of the photodiode may decide the quality (e.g., sensitivity) of the BSI image sensor, wherein the transforming efficiency is decided by the doping regions of the N-implanted layer and the P-implanted layer of the photo-diode. Therefore, to further exploit advantages of the BSI structure, more and more effort is dedicated to improve an area efficiency and compactness of a BSI image sensor.